Expose all metadata classes at package level

Author: michaelosthege

mcbackend/__init__.py

@@ -2,7 +2,8 @@
 A framework agnostic implementation for storage of MCMC draws.
 """
 from .backends.numpy import NumPyBackend
-from .core import Backend, Chain, ChainMeta, Run, RunMeta
+from .core import Backend, Chain, Run
+from .meta import ChainMeta, Coordinate, DataVariable, ExtendedValue, RunMeta, Variable
 
 # Backends
 try: